1. Field of the Invention
The present invention relates to an arrayed waveguide grating to be used for optical communications.
2. Description of the Related Art
Recently, in optical communications, as a method for significantly increasing the transmission capacity, dense wavelength division multiplexing system have been successfully researched and developed, and made practicable. In dense wavelength division multiplexing systems, a plurality of optical beams having wavelengths which are different from each other are multiplied for transmission. In such a dense wavelength division multiplexing system, a optical transmission element which transmits only a predetermined wavelength is required. This optical transmission element is used to extract optical beams by each wavelength from transmitted multiplexing optical beams at a light receiver side.
As an example of the optical transmission element, there is an AWG (Arrayed Waveguide Grating) as shown in FIG. 4, for example. In the arrayed waveguide grating, a waveguide structure is formed on substrate 1 as shown in the figure. That is, to the exit side of one or more optical input waveguides 2 , first slab waveguide 3 is connected. To the exit side of the first slab waveguide 3, a plurality of arrayed waveguides 4 are connected, and to the exit side of the arrayed waveguides 4, second slab waveguide 5 is connected. To the exit side of the second slab waveguide 5, one or more optical output waveguides 6 are connected.
The arrayed waveguides 4 transmit optical signal led out from the first slab waveguide 3, and are formed so as to be different in length from each other. The lengths of the adjacent arrayed waveguides 4 are different by xcex94L, and by these arrayed waveguides 4, diffraction grating 14 is formed.
Also, the optical input waveguides 2 and the optical output waveguides 6 are provided so as to correspond to the number of signal beams which are divided or synthesized by the arrayed waveguide grating and are different in wavelength from each other. The arrayed waveguides 4 are normally provided in a large number, for example, 100. However, in the figure, for simplification, the numbers of the optical input waveguides 2, arrayed waveguides 4, and optical output waveguides 6 are roughly shown. In the same figure, although the arrayed waveguides 4 are shown as arc shapes, however, in actuality, the center part in the length direction of the arrayed waveguides 4 is formed to be more linear than in the figure.
To the optical input waveguides 2, for example, an optical fiber (not illustrated) at the transmission side is connected, whereby wavelength multiplexing signal beams are led-in. Signal beams led-into the first slab waveguide 3 through the optical input waveguides 2 are spread by the diffraction effect, made incident onto the arrayed waveguides 4, and transmitted through the arrayed waveguides 4.
The signal beams transmitted through the arrayed waveguides 4 reach the second slab waveguide 5, and furthermore, converged onto the optical output waveguides 6 and outputted. However, since all lengths of the arrayed waveguides 4 are different from each other, phase differences between individual signal beams transmitted through the arrayed waveguides 4 occur, the phasefront of the optical signal inclines in accordance with the amount of phase differences, and due to the inclination angle, the light focus position is determined. Therefore, the light focus positions of light beams with varying wavelengths are different from each other, and by forming optical output waveguides 6 at the positions, signal beams which are different in wavelength can be outputted from the individual optical output waveguides 6 by each wavelength.
Furthermore, since the arrayed waveguide grating uses the principle of optical reversibility, the grating has a function as a spectral synthesizer as well as a function as a spectral divider. That is, if a plurality of signal beams having different wavelengths are made incident from the optical output waveguides 6, respectively, these signal beams pass through the transmission path opposite to the above path, and are synthesized by the arrayed waveguides 4, and then exit from the optical input waveguides 2.
In such an arrayed waveguide grating, as mentioned above, the wavelength resolution of the diffraction grating is in proportion to the difference (xcex94L) in length between the arrayed waveguides 4 comprising the diffraction grating. Therefore, by designing xcex94L to be large, spectral synthesis and division of wavelength multiplexing signal beams with narrow wavelength intervals become possible although they are impossible in a prior-art diffraction grating. That is, the arrayed waveguide grating can show spectral synthesis and division functions for a plurality of signal beams (a function to divide or synthesize a plurality of optical signals with wavelength intervals of 1 nm or less) required to realize high density dense wavelength multiplexing systems.
Among prior-art arrayed waveguide gratings, some arrayed waveguide gratings divide or synthesize optical signals of 16 wavelengths which are different from each other and have wavelength intervals of, for example, 100 GHz. A design example of such an arrayed waveguide grating shall be described. In this arrayed waveguide grating, the FSR (Free Spectral Range) is 25 nm, the focal lengths of the first and second slab waveguides 3 and 5 are 11.9 mm, respectively, the diffraction order is 61, and xcex94L=65.2 xcexcm. Also, in this arrayed waveguide grating, the aligning pitch of the arrayed waveguides 4 at the center in the length direction thereof is 25 xcexcm, and the aligning pitch of the arrayed waveguides 4 at each connection point to the first and second slab waveguides 3 and 5 is 20 xcexcm.
In this design example, since the number of the arrayed waveguides 4 is set to 200, and the aligning pitch of the arrayed waveguides 4 is 25 xcexcm at the center in the length direction thereof, the width of the arrayed waveguide forming range (A in FIG. 4) at the center in the length direction of the arrayed waveguides 4 is approximately 5 mm (19xc3x9725 xcexcm=4.975 mm).
In the above arrayed waveguide grating, it is generally known that, since the TE mode transmission speed and TM mode transmission speed for light beams being transmitted through the arrayed waveguides 4 are different from each other, polarization dependence loss occurs.
Therefore, in order to eliminate this polarization dependence loss, as shown in FIG. 4, in the prior-art arrayed waveguide grating, a half-wavelength plate 8 (8a) is provided so as to be across the center in the length direction of all arrayed waveguides 4. The half-wavelength plate 8 (8a) is inserted into slit 7 which is formed so as to be orthogonal to the arrayed waveguides 4 at the center in the length direction of the waveguides.
The half-wavelength plate 8 (8a) functions as a polarization mode conversion part which converts the TE mode and TM mode of light beams to be transmitted through the arrayed waveguides 4. If the half-wavelength plate 8 (8a) is provided at the center in the length direction of the arrayed waveguides 4, before and after the half-wavelength plate 8 (8a), the TE mode is converted to the TM mode, or the TM mode is converted to the TE mode. By this conversion, the difference in transmission speed between the TE mode and TM mode occurring before optical beams transmitted through the arrayed waveguides 4 are made incident onto the half-wavelength plate 8 (8a) is canceled until the optical beams are transmitted to the exit side of the arrayed waveguides 4, whereby polarization dependence loss is eliminated.
The half-wavelength plate 8 (8a) is provided so that the vertical length (B in the same figure) corresponds to the width of the arrayed waveguide forming range (A in the same figure) at the center in the length direction of the arrayed waveguides 4. In the arrayed waveguide grating of the abovementioned design example, as mentioned above, the width A of the arrayed waveguide forming range at the center in the length direction of the arrayed waveguides 4 is approximately 5 mm. Therefore, in the arrayed waveguide diffraction grating of the abovementioned design example, the half-wavelength plate 8 (8a) whose length shown by B in the figure is 7 mm is inserted, adhered, and fixed.
If the number of multiplexing wavelengths increases in dense wavelength division multiplexing systems, it is required to increase the number of wavelengths to be divided and the number of wavelengths to be synthesized by the arrayed waveguide grating to be used as a optical transmission element in accordance with the increase. In the case where the number of wavelengths to be divided or synthesized by the arrayed waveguide grating is increased, in terms of designing, the number of arrayed waveguides 4 increases.
However, in the arrayed waveguide grating in which the number of arrayed waveguides 4 is large, the width of the arrayed waveguide forming range (A in FIG. 4) at the center in the length direction of the arrayed waveguides 4 becomes wider. Therefore, although the length of the half-wavelength plate 8 provided in accordance with the width of the arrayed waveguide forming range must also be increased, at present, the length of the half-wavelength plate 8 which has been generally used is approximately 5 through 10 mm. And, a half-wavelength plate 8 whose length is longer than that of the general plate is an article specially made to order, which is expensive, and therefore, an arrayed waveguide grating with a longer half-wavelength plate provided also becomes expensive.
For example, the designed values of specifications of the arrayed waveguide grating for dividing or synthesizing optical signals of 32 wavelengths which are different from each other and have wavelength intervals of 100 GHz are as follows. That is, the FSR is 51 nm, the focal lengths of the first and second slab waveguides 3 and 5 are 24.3 mm, respectively, diffraction order m is 30, xcex94L=32.1 xcexcm, and the number of alignments of the arrayed waveguides 4 are 406. The aligning pitch of the arrayed waveguides 4 at the center in the length direction thereof is 25 xcexcm, and the aligning pitch of the arrayed waveguides 4 at each connection point to the first and second slab waveguides 3 and 5 is 20 xcexcm. Therefore, as shown in FIG. 5, the width of the arrayed waveguide forming range (A in FIG. 5) at the center in the length direction of the arrayed waveguides 4 is increased to be 10.1 mm (405xc3x9725 xcexcm=10.1 mm).
If so, approximately 12 mm is required as the length B of the half-wavelength plate 8 (8t) to be inserted into the center in the length direction of the arrayed waveguides 4, however, the half-wavelength plate 8 (8t) with a length of approximately 12 mm is an article specially made to order, which is expensive. Furthermore, it is extremely difficult to precisely insert the half-wavelength plate 8 (8t) with a length of 12 mm into the arrayed waveguides 4, and when inserting the half-wavelength plate, it may bend or take-in bubbles in an adhesive, and therefore, crosstalk becomes worse such as approximately xe2x88x9223 dB by roughly 10 dB.
The present invention is made in order to solve the abovementioned problems.
In an arrayed waveguide grating which is arranged so that a waveguide structure is provided in which a first slab waveguide is connected to the exit side of one or more optical input waveguides, a plurality of arrayed waveguides which have varying lengths to transmit optical signal led out from the first slab waveguide are connected to the exit side of the first slab waveguide, a second slab waveguide is connected to the exit side of the plurality of arrayed waveguides, and a plurality of optical output waveguides are connected to the exit side of the second slab waveguide, a plurality of optical signals which are inputted from the optical input waveguides and are different in wavelength from each other are transmitted by being provided with phase differences for each wavelength by the arrayed waveguides, and made incident onto individual optical output waveguides for each wavelength, whereby light beams with varying wavelengths are outputted from the light output waveguides, wherein a polarization mode conversion part to converts the TE mode and TM mode of light to be transmitted through the arrayed waveguides is provided so as to cross the middle portion in the length direction of all arrayed waveguides, and said mode conversion part is formed by providing a plurality of half-wavelength plates in series in the vertical direction of the arrayed waveguides so as to cross the plurality of arrayed wave guides, and a plurality of arrayed waveguides crossed by one of the half-wavelength plates is defined as one arrayed waveguide group, and based on this, arrayed waveguide groups of a number corresponding to the number of disposed half-wavelength plates are formed. Advantageously, the interval between adjacent arrayed waveguide groups at the disposed portion of the half-wavelength plates is set to 50 xcexcm or more.